Today I learned there are multiple BTU definitions. https://en.wikipedia.org/wiki/British_thermal_unit lists Thermochemical, 59 °F, 60 °F, 39 °F, and International Steam Table.

Though as the difference is at most 0.5%, it's probably won't affect your battery buying experience. :)

Measuring by TNT equivalent is more standardized. "This battery stores 50 grams of TNT."

Ummm, on second thought, maybe don't use that term at the airport, .. or in secure areas, ... or near the police, ... or in public, ... or on social media or anything else tapped by the NSA or other authorities.